1. Field of the Invention
The present invention relates to a communication network and method for transmitting data packets in the communication network which comprises synchronized nodes via a predetermined path in the communication network.
2. Description of the Related Art
Communication networks for control and regulatory processes, as in the manufacturing and processing industry, are often physically distributed systems in which data need to be transmitted between the participating nodes with a particular quality of service. Examples of such nodes are controllers, sensors, actuators or switches. The communication platform typically used is an industrial Ethernet with realtime capability, such as PROFINET. To ensure short end-to-end delays and little variation therein, particularly jitter, the critical control and regulatory data need to be forwarded with preference, i.e., particularly with priority, over other data in the communication network.
The aforementioned PROFINET communication network as an example of an industrial Ethernet standard is based on a time-division multiplexing method for ensuring different qualities of service. In this case, all the nodes of the PROFINET communication network are synchronized to one another. A time cycle of the time-division multiplexing method used comprises fixed time windows for isochronous realtime traffic (IRT, Isochronous Realtime), other realtime traffic (RT, Realtime) and normal traffic. This cycle is continually repeated. Within the respective time window, exclusively the traffic intended therefor is attended to. Other packets are buffer-stored at the respective node.
The time window for isochronous realtime traffic allows deterministic transfer characteristics, such as for motion control applications. In this context, the network resources along the scheduled path in the communication network are exclusively reserved even before the system is started up. Hence, there is a precise stipulation of when and on what network interface or port a node feeds in a packet or forwards a received packet. This approach allows the prevention of competing access to network resources, such as the simultaneous forwarding of packets via the same port of a node. However, the available resources of the communication network cannot be jointly used by other traffic dynamically and hence efficiently.
In the time window for normal traffic, on the other hand, packets are forwarded based on their priority, particularly via strict priority scheduling, and static multiplexing. Packets are forwarded only when they have been received completely. This can be called store and forward forwarding. However, this can entail high-priority control and regulatory data packets experiencing waiting times at the nodes because the relevant port to which the packets are intended to be forwarded is already blocked by an ongoing transmission process for another packet. This can result in greatly varying end-to-end delays. This problem is shown schematically in FIGS. 1 and 2. In this case, FIG. 1 illustrates a schematic diagram of a network topology for a communication network N with four nodes A, B, C, D. A path P of the communication network N is determined by the subpaths D-C, C-B and B-A. In addition, FIG. 2 shows a schematic diagram for the transmission of data packets P1-P3 via the communication network N of FIG. 1.
The packets P1 and P3 are high-priority data packets, whereas the packet P2 is a low-priority data packet. In this case, by way of example, the node A is a control device (controller) to which high-priority data packets, in this case the data packets P1 and P3, are transmitted cyclically from the sensor nodes B and D. By way of example, the node C is an actuator that receives high-priority manipulated values cyclically from the control device A. Here, the node C sends only low-priority data packets P2 to the node A. Furthermore, it can be assumed that the high-priority data packets P1 and P3 are transmitted as Ethernet packets having the same length. FIG. 2 considers exclusively the communication in the direction of the node A, the destination node. However, a corresponding situation also applies to the opposite direction, in that case to the destination node D.
In detail, FIG. 2 shows the message flow diagram for the transmission of the data packets P1-P3 at the beginning of a cycle at the instant t=0. In this case, t shows the temporal extent, whereas R illustrates the spatial extent. At the instant t=0, the node B sends the high-priority data packet P1 in the direction of the node A, the node C sends the low-priority data packet P2 in the direction of the node A and the node D sends the high-priority data packet P3 in the direction of the node A. Each node B, C, D has only one data packet P1, P2, P3 ready for sending in the direction of the destination node A. As a result, the respective data packet P1, P2, P3 is transmitted at each node B, C, D. The problem is the long transmission period for the low-priority data packet P2 at the node C on account of the high packet length thereof, particularly in comparison with the packet lengths of the high-priority data packets P1, P3.
At the node C, the high-priority data packet P3 arriving from the adjacent node D must wait for the complete transmission of the low-priority data packet P2, since the transmission process for the low-priority data packet P2 had already been started before the packet P3 arrived at the node C. Only after complete transmission of the low-priority data packet P2 with the relatively high data length can the data packet P3 be transmitted from the node C to the node B. At the node B, the data packet P3 must also then await the complete transmission of the data packet P2 from the node B to the node A. The result of this is a long delay for the high-priority data packet P3, particularly for long linear topologies comprising a plurality of nodes.